Polyolefin films are widely used as packaging films. The success of these materials is based on the good optical and mechanical properties and on the ease of welding of the films. In addition to welding, heat-sealing of films has increased in importance. Heat-sealable films preferably have a top layer of a polymer with a lower crystallite melting point than the polymer of the base layer of the film. For sealing, the film layers are laid one on top of the other and warmed only to from 10 to 20 C. below the crystallite melting point, i.e. the top layers are not completely melted. The adhesion achieved between the heat-sealing layers is significantly less than if the same material is welded, but is sufficient for many applications (Kunststoff-Handbuch [Plastics Handbook], Volume IV, Carl Hanser Verlag, Munich, 1969, pages 623 to 640).
The seal seams in many cases have greater mechanical strength than the films themselves, so that opening of a sealed film package results not only in the latter being torn and destroyed in the seal seam, but rather in the tear continuing in the film itself and then tearing further in an uncontrolled manner. For this reason, heat-sealable raw materials are also replaced by peelable top layers applied to film surfaces. The peelable top layers offer firstly good heat-sealing properties, but at the same time also offer the possibility of opening the seal seam again in a controlled manner without destroying the sealed materials. This opening of the seal seam takes place with cohesive fracture in the peel layer, which ideally take place only in the peel layer.
Peelable raw materials are known from the prior art. For example, the product datasheet "Novolen VP 9201" describes an olefinic polymer which is suitable for the production of peelable, coextruded heat-sealable layers on polypropylene films. The heat-sealing/peel layers are glossy to silk-matt and are predominantly used on pigmented/opaque support layers.
U.S. Pat. No. 4,666,778 describes transparent, peelable films having good seal seam strength and low haze. The peelable top layer comprises a polymer mixture of ethylenic polymers or ethylenic copolymers containing small amounts of propylene polymers and butylene polymers. The good transparency of the films is favorable for some applications, but is not always desired.
The prior art also discloses nontransparent, i.e. opaque or translucent films, which, depending on the top layer applied, can be heat-sealable or non-heat-sealable.
Opaque films contain in at least one layer pigments or vacuole initiating particles or a combination thereof, causing the films to have significantly reduced light transmission compared with transparent films. For the purposes of the present invention, "opaque film" means a nontransparent film whose light transparency, measured in accordance with ASTM D 1003-77, is at most 70%, preferably at most 50%.
Vacuole initiating particles are particles which are incompatible with the polymer matrix and result in the formation of vacuole-like cavities when the films are stretched, the size, type and number of the vacuoles depending on the material and on the size of the solid particles and on the stretching conditions, such as stretching ratio and stretching temperature. The vacuoles reduce the density and give the films a characteristic mother-of-pearl-like, opaque appearance caused by light scattering at the vacuole/polymer matrix interfaces. In general, the mean particle diameter of the vacuole initiating particles is from 0.02 to 10 .mu.m.
EP-A-0 083 495 describes an opaque, biaxially oriented film having a satinized appearance and a surface gloss of greater than 100% and containing at least one spherical solid particle per vacuole. In addition, the film has on both surfaces of the core layer a pore-free, transparent, thermoplastic cladding layer which has a thickness which determines the appearance of the film. An example given of a material for the solid particles is nylon. The particles generally have a diameter of greater than 1 .mu.m. In this film too, the opacity is determined principally by the amount of solid particles and the process parameters during biaxial orientation.
EP-A-0 475 110 describes biaxially oriented white films having a support layer based on polypropylene polymers and top layers on one or both sides. The support layer contains coated TiO.sub.2 particles whose coating contains at least one inorganic oxide and at least one organic compound. The films are distinguished by good mechanical properties and a high, permanent degree of whiteness.
EP-A-0 538 747 describes biaxially oriented polypropylene films having a matt top layer. The top layer comprises propylene copolymers and/or terpolymers and an HDPE. The films are distinguished by low gloss and high haze, ensuring the matt appearance.
EP-A-0 367 613 describes an opaque film having a vacuole-containing layer and a top layer having a rough, inscribable surface. The top layer contains a first polymer having an MFI of 1 g/10 min and a second polymer which is incompatible with the first.
U.S. Pat. No. 5,500,265 discloses a peelable film comprising the invention relates to a peelable film comprising an olefinic base layer and a skin layer on at least one surface of thge base layer, the skin layer comprising a blend of a butylene polymer with another olefin polymer or a polymer of butylene and at least one other olefin and a coating layer on the skin layer. Said coating layer is a heatsealable acrylic polymer or a polyvinylidene polymer. Said film is disadvantagous in that it cannot be recylced in the production process due to the acrylics or PVC contained. Also its production is expensive because the coating is applied in a separate step after the production of the film and for most coatings it is necessary to apply a primer between the skin layer and the coating. The film peels between the skin layer and the coating. Thereby residues of the coating remain on the substrate after peeling, which is specifically undesired if the film is used on yoghurt cups.
It has been found that the opaque carrier films are only of limited suitability for application of peel layers. Opaque films with peel layers have major disadvantages in certain applications. In particular when used as lid films for food containers, problems occur that are connected with the vacuole-containing structure. After removal from the container, the lid film leaves behind white shred-like skins on the embossing rim of the container on to which the film has been welded. It was found during investigations concerning the present invention that these film residues on the container rim are caused by the poor tear propagation strength of the film. On removal of the lid film, the peel layer tears as intended, but this tear then propagates in an uncontrolled manner in the other layers of the film. This leaves film residues adhering to the container rim which cannot easily be identified by the consumer and can under certain circumstances cause nausea. For marketing reasons, such effects are unacceptable. The known opaque films having peel layers can therefore hardly be used for this application.
It is known that the tear propagation strength of opaque, peelable films can be increased by only pigments which do not form vacuoles as fillers for opacifying the film. This teaching is described in the European Application with the Application No. P 44 24 604.8-16. However, it has been found that this measure is inadequate. In spite of the fact that the internal strength of this film is greater than in a film containing vacuoles, the tear propagation strength is inadequate always to prevent the skin formation described when used as a lid film.
During investigations concerning the present invention, it was found that the skin formation can be avoided if the peel layer is applied in a thickness of greater than 5 .mu.m. However, this solution to the problem is not advantageous, since the peel material is very expensive. Thick layers of peel raw material makes the film so expensive that it can no longer be offered at a commercial price. In addition, thick peel layers make it very difficult to match the seam strengths to various applications.